Husky MP2500 Manual page 324

Each cause can be enabled or disabled independently under program
control, with the exception of hard reset and the P key. A program can also
determine the reason for power-up, so it can respond appropriately. For
example, a program can conserve power by turning off (see the next
section) while waiting for a call or the correct time. (See section 32.3 for
more information on saving power.)
32.1.1.1 Power-up state
When turned on, MP2500 will normally resume execution from the state at
which it was turned off. This is called 'resume mode', and helps make
portable operation extremely convenient.
You can cancel resume mode by using INT 15H, function 64H, and
MP2500 will behave more like a desktop computer, rebooting whenever it is
turned on. (Resume mode will also be cancelled if all battery power has
been exhausted, including the backup battery.)
The contrast level will normally power up set to a value suitable for normal
operating conditions, but you can set the power-up state by using INT 15H
function 75H.
32.1.1.2 Power-up key
The power-up key is P. This key cannot be disabled or redefined; if there
is any power in the batteries, it will always turn MP2500 on (although
MP2500 may turn off again immediately, for example if the battery power is
below the low power level).
The easiest way to check if P has caused the power-up is to enable the
power key event (using INT 15H function 6EH) before power down, then
check the event details on power up, using INT 15H function 6DH.
32.1.1.3 RI
RI is not available on a standard MP2500.
302 H MP2500 System Developer's Guide
USKY